Isotope dilution mass spectrometry as an independent assessment method for mass measurements of milligram quantities of aqueous solution

Abstract

Isotope dilution mass spectrometry was used to independently assess the accuracy of mass measurement methods developed to quantitatively dispense milligram quantities of aqueous solution. Solutions of isotopically enriched 158Gd and enriched 155Gd were prepared with the molality (mol g−1) of the 155Gd solution being approximately 400 times more dilute than the 158Gd solution. Aliquots of the 158Gd solution were flame sealed in glass ampoules and the 155Gd solution was quantitatively dispensed as a series of 1-g sample aliquots and larger (45 g–55 g) calibration aliquots. Calibration mixes of the solutions were prepared by mass and the Gd isotopic compositions of the 155Gd, 158Gd, and mixed Gd calibration solutions were measured using a high-resolution multi-collector inductively coupled plasma mass spectrometer. These data were used to calculate an elemental amount ratio for Gd in the solutions. Several 155Gd solution sample units were then spiked with small aliquots (0.25 mg–6 mg) of 158Gd solution for which the masses were determined using glass microcapillary and inkjet dispensing methods. The measured Gd isotopic compositions of these mixed solutions, along with the previously calibrated Gd elemental amount ratio and the known masses of the 155Gd aliquots, were used to calculate masses of dispensed 158Gd solution samples. These isotope dilution-based mass measurements have relative expanded uncertainties <0.1% (k = 2), which will allow solution masses measured by microcapillary and inkjet dispensing methods to be assessed for accuracy.